1. Field of the Invention
The present invention relates to a beam spot controlling method for controlling the position onto which a spot of beam is projected on an optical recording medium such as an optical disk, a magneto-optical recording disk, and an optical card for use in an optical recording system in which data is optically written in and read out by spot irradiation, and an apparatus or actuator therefor.
2. Discussion of Background
In an optical recording system, it is necessary to use an objective lens to focus a laser beam and direct the beam onto the recording track of a recording medium in the form of a beam spot of minute diameter. For instance, in the case of a disk-shaped recording medium such as an optical disk and a magneto-optical recording disk (hereinafter referred to as the recording disk), the objective lens is moved in a radial direction of the recording disk as the recording disk is rotated, whereby the beam spot irradiation onto the desired recording track can be performed.
In a random access mode, the objective lens is first driven at high speed in such a manner that a laser beam gets rough access to the vicinity of the desired recording track, reading the track address, and getting fine access to the recording track, so that the laser beam performs spot irradiation on the desired recording track.
However, the width of the recording track is extremely small, and because there are limits to the precision of the shape of the recording track, to the positioning of the recording medium when it is installed, or to the driving of the objective lens, it is not possible to direct the light ray spot onto the recording track of the recording medium with any degree of accuracy by simply driving the objective lens mechanically. Accordingly, any deviation between the spot and the recording track position must be detected as an error signal, and the tracking direction must be finely controlled to direct the spot onto the recording track. In addition, it is also necessary to finely control the focusing direction so that the focal point ends up on the recording track. Various types of such tracking control systems are proposed, for instance, one for an optical information reading-out apparatus as described in U.S. Pat. No. 4,302,830.
FIG. 16 is a perspective view of a conventional beam spot control apparatus in the explanation of the movable operating parts thereof. FIG. 17 is a cross-sectional view of the movable operating parts, which are fitted by a sliding shaft. In these figures, an objective lens 13 is mounted on a supporting member 81 which is referred to as a bobbin. A focusing coil 25 is wound around the peripheral surface of the supporting member 81, and tracking coils 27 are also attached around the supporting member 81 as shown in FIGS. 16 and 17. When a signal current is caused to flow through the focusing coil 25 and the tracking coils 27, power is generated between the supporting member 81 and a separate electro-magnetic circuit (not shown), so that the supporting member 81, which is fitted with a sliding shaft 87 through a fitting hole 85, is turned around the sliding shaft 87 and accordingly the objective lens 13 is driven in a tracking direction Tr. In addition, the objective lens 13 is driven in a focusing direction Fo by the vertical sliding movement of the supporting member 81 along the sliding shaft 87.
To maintain the weight balance in this type of beam spot controlling apparatus, an equivalent weight balancer 83 is provided on the side opposite to the objective lens 13, but in actual manufacture, it is impossible to avoid the creation of a weight imbalance around the sliding shaft 87. For this reason, the supporting member 81 is subject to displacement in the tracking direction Tr as the result of acceleration during the high speed tracking access, so that vibrations are induced in the supporting member 81. Even during the fine access action, the thus induced vibrations remain, which cause a delay in the access time. In particular, because the objective lens 13 and the balancer 83, which are both heavy, are positioned at a location separated from the sliding shaft 87 which is the center of rotation, the moment of inertia is large and even a small weight imbalance tends to cause the objective lens 13 to swing. In addition, the balancer 83 is essentially a useless object, so that it is desirable to eliminate it or to provide a light unit to lighten the weight of the actuator.
Furthermore, as shown in FIG. 16, when tracking is controlled by moving the objective lens 13 horizontally, the problem arises that even when tracking is obtained, the reflected laser beam from the recording medium is displaced parallel to the incident laser beam and an apparent tracking error is produced. FIGS. 18A and 18B are explanatory figures showing this situation. FIG. 18A shows an objective lens O.L. at a neutral point and the center of the optical intensity distribution of an incident beam 101 coinciding with the optical axis of the objective lens O.L. Generally the center of the light intensity distribution of a beam is the center of the beam. Therefore, the center of the light intensity distribution of a beam is hereinafter simply referred to as the center of the beam or the center of an incident beam. In the case as shown in FIG. 18A, the center of the incident beam 101 exiting from a laser source coincides with the center of a beam 103, which is focused by the objective lens O.L. and reflected from the surface of the recording disk. The reflected beam 103 is reflected by a mirror M, passes through a beam splitter B.S. and enters a detection system including a quadrant photodiode, in which error signals and data signals are detected. The reflected beam 103 enters the center of the quadrant photodiode P.D., and when tracking is obtained, no bias is produced in the amount of beam directed to the quadrant photodiode P.D. because the intensity of beam is evenly distributed from the center of the laser beam. If the tracking deviates from the center of the track, a deviation is produced from the center of the light intensity distribution, and the amount of the deviation is detected as an error signal. Here it is supposed that the objective lens O.L.. is displaced by a distance, .DELTA.x, from the neutral position, for instance, to compensate for the deviation of the recording track, and the beam-converging position on the disk is moved by the distance .DELTA.x only. At this time, the incident beam 101 enters at a position with its center shifted by the distance .DELTA.x, from the optical axis of the objective lens O.L., so that the center of the incident beam 101 and the center of the reflected beam 103 remain parallel, but separate by a distance .DELTA.L.sub.2 (=2.DELTA.x). Therefore, the center of the reflected beam 103 is deviated from the right center by this distance and enter the quadrant photodiode. The result is that in spite of the fact that accurate tracking has been obtained with an equal light intensity distribution from the center of the laser beam and spot formation has been performed correctly on the recording track, the detector reacts as though a tracking error has occurred. If the beam spot control is performed in such a situation, the beam spot is formed at a position shifted from the center of the recording track. Accordingly, if the amount of this apparent tracking error is not offset, the quality of the recording and reproduction of the information is significantly reduced.
In order to compensate the amount of the apparent tracking error, a method has been proposed in Japanese Laid-Open Patent Application 58-9228 by which a beam is directed to an objective lens, and the reflection surface of a galvanomirror which directs the reflected beam to a tracking error detection system is made to coincide with the back focal point of the objective lens. However, since the back focal distance of the objective lens is extremely small, for instance, about 2 to 5 mm, it is almost impossible to design the system to place the galvanomirror at that position.
In Japanese Laid-Open Patent Application 61-160841, a method is disclosed by which a rotary prism acting as a mirror is driven eccentrically so that the light beam is always passes through the back focal point of the objective lens. However, with this method a large apparatus is unavoidable.
In addition, in these two methods it is difficult to precisely maintain the attitude of the mirror when an optical head accesses the recording disk at high speed.
The use of the property of a tapered prism that the inclination of the light beam exiting therefrom can be changed by turning the prism has been proposed in order to scan a light beam or to change the optical axis of a light beam, for instance, in U.S. Pat. Nos. 2,975,668, 3,297,395, 3,378,687, 3,736,848, 3,827,787, and 4,118,109. However, these prior art references neither teach or suggest the application of that property to a beam spot irradiation apparatus.